The present invention relates to chromatic dispersion measuring method and apparatus for measuring the chromatic dispersion characteristic of optical signals which are transmitted over optical transmission lines such as an optical fiber and an optical amplifier repeater system having multiple stages of optical fibers connected via optical amplifiers.
The chromatic dispersion characteristic of an optical fiber is obtainable from the degradation of the propagation time relative to the wavelength of an optical signal, that is, from the group delay characteristic, and it is usually expressed by the propagation time difference per unit wavelength.
In a fiber optic communication using an optical fiber as a transmission line, the waveform of the optical signal after transmission is distorted owing to the relationship between spreading of the wavelength of the optical signal and the chromatic dispersion characteristic of the optical fiber, so that this leads to degradation of the receiving characteristic. Moreover, in an optical amplifier repeater system, the chromatic dispersion characteristic of optical fibers forming the system is accumulated, and hence a non-linear effects of the optical fibers by such a chromatic dispersion characteristic seriously affect the transmission characteristic. It is essential, therefore, for the construction of an optical communication system to study the chromatic dispersion characteristic in detail.
FIG. 9 is a block diagram showing a first conventional light chromatic dispersion measuring apparatus.
In FIG. 9, reference numeral 1 denotes an oscillator for generating a single-frequency signal, 2 a tunable-wavelength light source formed by a plurality of light sources of different wavelengths or a tunable-wavelength light source formed by a single light source but capable of oscillating at a plurality of wavelengths, 3 an optical divider for splitting the output from the tunable-wavelength light source 2, 4 a reference optical fiber, 5 an optical transmission line to be measured, such as an optical fiber or optical amplifier repeater system having one or more stages of optical fibers connected via optical amplifiers, 6 a light receiver or photodetector for converting the optical signal output from the reference optical fiber 4 to an electrical signal, 7 an optical receiver or photodetector for converting the optical signal output from the optical transmission line 5 to an electrical signal, and 8 a phase comparator for mutually comparing the phases of the electrical signals output from the photodetectors 6 and 7.
The oscillator 1 supplies to the tunable-wavelength light source 2 a single-frequency signal which is used for intensity modulation of the optical signal output therefrom. The intensity-modulated optical signal from the tunable-wavelength light source 2 is split by the optical divider 3. The one of such split optical signals is used as a reference for phase comparison; it is provided to a reference input of the phase comparator 8 via the reference optical fiber 4 of a short length and the photodetector 6. The other optical signal is input into the phase comparator 8 via the optical transmission line 5 and the photodetector 7. The phase comparator 8 detects a phase difference between two input optical signals. By performing this for each wavelength, the group delay characteristic can be obtained.
That is, the relative propagation time .tau.(.lambda.) of light of a wavelength .lambda. in the optical transmission line 5 is obtain-able from the measured phase difference .theta.(.lambda.) as follows: EQU .tau.(.lambda.)=.theta.(.lambda.)/360f
where f is the oscillation frequency of the oscillator 1. By converting them to unit distance (km) and representing the abscissa with the wavelength .lambda. and the ordinate with .tau.(.lambda.), the group delay characteristic can be obtained, and by differentiating this .tau.(.lambda.) with the wavelength .lambda., the chromatic dispersion characteristic can be obtained.
The chromatic dispersion measuring apparatus of Prior Art Example 1 involves sequential changing of the wavelength for measurement, and hence needs some amount of measuring time; in this instance, however, an ambient temperature change during a measurement operation causes a change in the length of the optical fiber used as the optical transmission line 5, inducing an error in the measurement of the relative propagation time .tau.(.lambda.).
There has been proposed a solution to the problem of elongation and shrinkage of the optical fiber by the ambient temperature change (Japanese Pat. Pub. No. 54292/91 entitled "Method for Measuring the Dispersion Characteristic of Optical Fibers"). It will herein below be described as Prior Art Example 2.
FIG. 10 is a block diagram illustrating a second conventional light chromatic dispersion measuring apparatus.
In this example, the parts corresponding to those in Prior Art Example 1 are identified by the same reference numerals and no description will be given of them. In FIG. 10, reference numeral 9 denotes a reference light source for emitting a reference signal for phase comparison use which has an optical signal wavelength different in region from the optical signal wavelength of the tunable-wavelength light source 2, 10 an optical coupler for coupling the output light from the tunable-wavelength light source 2 and the output light from the reference light source 9, and 11 a wavelength selection typed light splitter for splitting the optical signals from the tunable-wavelength light source 2 and from the reference light source 9 in their respective wavelength regions.
The oscillator 1 supplies to the tunable-wavelength light source 2 and the reference light source 9 a single-frequency signal for intensity modifying their optical signal outputs. The intensity-modulated optical signals from the tunable-wavelength light source 2 and the reference light source 9 are coupled together by the optical coupler 10. The two optical signals thus coupled are both input into the optical transmission line 5 and propagate therethrough. The optical signals having thus propagated through the optical transmission line 5 are separated by the wavelength selection typed optical divider 11 into the optical signal from the tunable-wavelength light source 2 and the optical signal from the reference light source 9. The thus separated optical signal from the reference light source 9 is provided via the photodetector 6 to a reference input of the phase comparator 8 for use as a signal reference in phase comparison. The optical signal from the tunable-wavelength light source 2 is provided via the photodetector 7 to the phase comparator 8. The phase comparator 8 detects a phase difference between the two optical signals. By performing this for each wavelength, the group delay characteristic can be obtained.
For example, in case of measuring a 1.55 .mu.m chromatic dispersion characteristic of an optical fiber, the oscillation wavelength of the tunable-wavelength light source 2 is set to a 1.55 .mu.m band, whereas the oscillation wavelength of the reference light source 9 is set to an entirely different 1.3 .mu.m band. By setting the wavelength of the reference signal to such a wave-length band different from that of the optical signal from the tunable-wavelength light source 2, the two optical signals can be separated by the wavelength selection typed optical divider 11, and the phase comparison can be made by using the 1.3 .mu.m band optical signal as the reference signal for phase measurement. Since the two optical signals propagate through the same optical fiber, this prior art example is free from the measurement error which results from elongation and shrinkage of the optical fiber.
As described above, according to Prior Art Example, 1, since the reference signal light and the signal light to be measured propagate through different paths, the elongation or shrinkage of the optical fiber for the light to be measured, by an ambient temperature change, seriously affects the time for propagation of the signal light through the optical fiber, impairing the accuracy of measurement of the chromatic dispersion characteristic.
On the other hand, Prior Art Example 2 avoids the influence of elongation and shrinkage of the optical fiber by simultaneously propagating the reference signal light and the signal light to be measured through the same optical fiber for the light to be measured. With this method, however, it is necessary that the optical transmission line has a wavelength bandwidth wide enough for transmitting signal components which are sufficiently spaced apart in wavelength to permit the separation of the two optical signals in terms of wavelength which is the feature of the above-mentioned prior art example. Hence, measurement for a single optical fiber can be made, but in an optical amplifier repeater system which has one or more stages of optical fibers connected using light amplifiers of limited transmission wavelength bandwidth, it is very difficult to transmit the reference signal light and the signal light to be measured and separate them at the receiving end, because the light wavelength bandwidth available for transmission is limited. For instance, in a case of a distance corresponding to an optical amplifier repeater system intended for transpacific communication, the optical wavelength available for transmission is as narrow as 2 to 3 nanometers; therefore, it is extremely hard to put the method of Prior Art Example 2 to practical use.
Furthermore, in such an optical amplifier repeater system, the chromatic dispersion characteristics of respective optical fibers forming the system are accumulated; hence, to keep its overall transmission characteristic excellent, it is necessary to measure the chromatic dispersion characteristic of each optical fiber with high precision and perform feedback processing such as equalization of the chromatic dispersion characteristic of the optical repeating system on the basis of the results of measurement.
However, there have not been proposed yet a method and apparatus for measuring the chromatic dispersion characteristic of an optical transmission line which sufficiently satisfy such requirements as mentioned above.